The present invention relates to optical memory systems, and more particularly, to an improved servo positioning and tracking system for accurately moving the beam of an optical disk storage device in a closed loop manner from one track to an adjacent track on the disk.
In a typical "write once read mostly" (WORM) optical memory system a laser beam is modulated by a pulse signal from an external information source and is recorded in binary form on the surface of a disk made of a special media. This is accomplished by physically altering small regions of the media arranged in concentric or spiral tracks. Such physical alteration may be in the form of heating to cause the formation of minute bubbles. The information recorded can be read by reflecting a laser beam off the surface of the disk at a lower power so as not to physically alter the small regions. See for example U.S. Pat. No. 4,466,087 of Cheng assigned to Xerox Corporation and entitled "Optical Memory System for a Reading/Writing, Verifying and Tracking Module."
By way of example, in one commercially available optical disk drive utilized as a peripheral storage device for a computer system, a twelve-inch disk has approximately 40,000 tracks resulting in a track density of 14,500 tracks per inch (tpi), a track pitch (distance between track center lines) of seventy micro-inches and a track width of twenty-four micro-inches.
In a typical optical disk drive utilized for data storage, the disk is grooved similar to laser video disks. The disk is impressed with radial servo-tracking information, data-synchronization signals and pre-formatting to provide radial head positioning feedback. The head positioning servo must be capable of accurately following the tracks and accommodating groove non-concentricity due to spindle run out, media wear and media errors.
When the typical optical disk drive performs a seek, a coarse actuator and a fine positioner actuator direct the read/write head. The coarse actuator is similar to the rotary voice-coil positioners used in some magnetic disk drives. The fine positioner, a small voice-coil actuator, is mounted on the coarse actuator. It moves the final focus lens radially across the tracks as well as up and down for focus control. When a long seek is initiated, an optical scale on the coarse positioner provides positioning feedback to the coarse positioner to bring the read/write head within the access range of the fine positioner. The fine positioner servo loop is then closed to acquire a track, read an address from the disk, determine the exact track position and compute a correction factor (the difference between the located address and the desired address). The fine positioner then locates the proper track. See for example U.S. Pat. No. 4,627,039 of Meyer assigned to Magnetic Peripherals, Inc. and entitled "Head Positioning Servo System for Optical Recording with Coarse and Fine Control." See also U.S. Pat. No. 4,627,038 of Abed et al. assigned to Storage Technology Partners II, and entitled "Optical Disk Storage Unit Having a Servo System with Different Velocity Inputs."
It is also possible to construct an optical disk drive without a coarse actuator. In such a drive all of the radial positioning of the read spot is accomplished by energizing a coil in the objective lens assembly.
Tracking information for optical disks can be derived from flags that are offset from true track center and are sampled as the disk rotates. A tracking error signal can be generated by taking the difference of the readback amplitude of these flags and applying that signal to the fine lens positioner to keep the read beam on track center. However, the difficulty arises when attempting to "micro-jump" to an adjacent track because the phase of the error signal is reversed when between track centers. In other optical disk drives that use the so-called "continuous far-field tracking technique" an error signal is continuously generated by monitoring fine tracking features on opposite sides of the data track groove. A tracking error signal is also generated which is applied to the fine positioner. Prior optical disk tracking techniques have opened the fine tracking loop and injected a fixed-length pulse. However, small variations in track pitch cause unreliable micro-jumps, resulting in poor system performance. Prior fine positioner systems have not accommodated small variations in track pitch, tracking gain, disk reflectivity or disk defects.